[Docs] Fix a code snippet about generating JSON xcontent (#46164)

Author: Hearen

docs/java-api/docs/index_.asciidoc

@@ -103,7 +103,7 @@ If you need to see the generated JSON content, you can use the
 
 [source,java]
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-String json = builder.string();
+String json = Strings.toString(builder);
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